The present invention is directed to a phase-difference detector for use with an auto-focus detecting apparatus of a camera.
A conventional auto-focus detecting apparatus utilizes a phase detector which correlates an arithmetic operation based upon signals outputted from pixels of a linear imaging device. This conventional auto-focus detecting apparatus is illustrated by FIG. 10 and will be described below.
As shown in FIG. 10, disposed behind a film-like surface 2 provided in rear of an imaging lens 1 are a condenser lens 3, a separator lens 4, and a phase-difference detector, respectively. The phase-difference detector comprises linear imaging devices 5 and 6 for optically receiving and photoelectrically converting a pair of images corresponding to a subject to be photographed. The images are formed by the separator lens 4. The phase-difference detector further comprises a processing circuit 7 for determining the focalization state of the images. This focalization state is determined with respect to electric signals generated by respective pixels in devices 5 and 6. These electric signals represent a distribution of luminous intensities.
Rays of light representing the images formed on the linear imaging devices 5 and 6 approach an optical axis 8 in a front defocus state where the image of the subject is positioned in front of film-like surface 2. However, in a rear defocus state, the rays of light which form the images move away from the optical axis 8. A predetermined position between the front and rear defocus states can be attained when the focalization state is determined. Hence, the processing circuit 7 functions to determine the focalization state by detecting the position where the rays of light are closer to the optical axis 8 on the basis of the electric signals generated by the imaging devices 5 and 6.
The detection of the relative positions of the images formed on linear imaging devices 5 and 6 involves the use of a phase-difference detecting method. Based on this method, correlative values of a pair of images formed on the linear imaging devices 5 and 6 are obtained by the arithmetic operation utilizing the following formula, ##EQU1## where L is the integral variable, e.g., 1 to 9, corresponding to an amount of relative movement of the focusing lenses.
The focalization state is determined according to the amount of relative movement (phase-difference) of the focusing lenses till the computed correlative value realizes a minimum value (or a maximum value).
The symbol B(K) represents a signal outputted in time-series from each pixel of one linear imaging device 5, while R(K-L-1) designates a signal outputted in time-series from each pixel of another linear imaging device 6. Correlative values H(1), H(2), . . . , H(9) are obtained by performing the arithmetic operation utilizing formula (1) every time the movement quantity L is varied from 1 to 9. For instance, as illustrated in FIG. 11, assuming that the focalization state is previously arranged to be preset when the correlative value H(4) is the minimum as shown in FIG. 11(a), and if the correlative value is at a minimum in such a position other than H(4), such as shown in FIGS. 11(b) and 11(c), the amount of deviation, i.e., a phase-difference when L=4, is detected as an amount of defocus.
In the above-mentioned conventional phase-difference detector, the correlation arithmetic operation is carried out with respect to the electric signals outputted from the respective pixels of the linear imaging devices. If the amplitude of the electric signal per pixel varies slightly, variations in individual correlative values H(1), H(2), . . . , H(L) become slight, resulting in that distinguishable features cannot be detected between the values. Namely, when the variations in respective correlative values are small, the maximum or minimum value cannot readily be ascertained in some cases. As a result, it is impossible to accurately detect the phase-difference from the focalization state.